DNA sits in sunlight every day, absorbing ultraviolet radiation that can set off the kind of chemical changes linked to mutations, aging, and cancer. Yet most of the time, the molecule avoids catastrophe. New simulations suggest that this resilience depends not on one built-in escape route, but on a crowded, ultrafast network of molecular reactions. These reactions can dump harmful energy almost as soon as it appears. The research focused on guanine and cytosine, two of DNA’s four chemical bases, arranged in short stacked segments that mimic key features of the double helix. By tracking their behavior at the atomic scale, the team found that once UV energy enters the system, the excited state quickly shifts into charge-transfer arrangements. Then, it relaxes through several competing pathways, many involving tightly linked movements of electrons and protons. That picture is more complicated than the simpler models that have often dominated the field. Instead of following a single preferred track, the excited DNA fragments explored multiple routes back to stability. Often, this happened within femtoseconds. Each one of …